Such masonry drill bits in particular are suitable for concrete or reinforced concrete, further for other masonry, rocks or the like. Especially high performance is required of such drill bits when drilling metal-reinforced concrete. The cutting geometry of the masonry drill bit precludes “controlled” chipping the reinforcement such as is carried out by metal cutting tools. Instead severing the reinforcement must be tried at an appropriate drill bit speed and the highest possible applied pressure. This procedure accordingly is a mixture of chipping and plastic deformation. The masonry drill bit therefore is highly loaded.
Substantial design criteria for the masonry drill bits being discussed here are simple centering, the attainable drilling rate (advance per unit time), long service life and lastly wear.
A known masonry drill bit (EP 0 761 927 B1) is fitted with a stamped hard metal point. In this manner and using a small contact area between the drill bit and the material being drilled, a predetermined pressure will result in high thrust. Such a shape also allows good centering. On the other hand, and in particular when drilling reinforced concrete, the said metal point must be expected to break off. Accordingly high thrust and good centering are traded against high wear.
Another masonry drill bit (EP 1 029 622 A2) also is fitted with a hard metal point which however is somewhat “countersunk” axially into the bit. In this manner the hard metal tip also is supported effectively in the drill bit and wear is reduced.
Both masonry drill bits discussed above incur the drawback that the chisel edge present at the center of rotation does not cut away in this zone. Consequently the “dead” zone of the masonry drill bit impedes the drilling advance.
A much different approach to eliminate the above impediment to drilling advance is disclosed in relation to a further known masonry drill bit in DE 199 44 406 C2. Essentially the drill bit of the last reference is fitted with a substantially roof-shaped insert. Lips run on both sides of the insert and are constituted by a leading lip and a trailing free surface. In this design, the radially outer ends of the chisel edge project axially at the end face of the insert relative to the in-between center of the chisel edge. In other words, the design might be termed being “an indented chisel edge”. This shall be the meaning of the expression in the discussion below.